The present invention relates to a roller of a relatively large size for a machine for manufacturing endless tape material, for example a paper machine or foil-pulling calender or mangle.
Rollers of the type under discussion are elongated rollers, which are comprised of a roller outer sleeve in which a push-away or displacement member is rigidly centrally positioned so that a cylindrical clearance is formed between the inner surface of the outer sleeve and the external surface of the displacement member. That clearance serves for passing therethrough a heat-conducting fluid to heat the roller. The roller is also provided with two trunnions laterally positioned on the sleeve and connected thereto. The trunnions are formed with a fluid-feeding passage and a fluid-discharging passage, respectively to admit and discharge heat-conducting fluid into and from the aforementioned clearance.
Rollers for machines for fabricating endless tape material usually have the length of 3 to 12 m and outer diameter of 30 to 120 cm. Since such rollers, in spite of required slenderness ratio, must be very rigid the wall of the outer sleeve should be sufficiently thick. Therefore the outer sleeves of such rollers are normally formed by mold-casting. Preferably the outer sleeve of the roller is made of a chilled cast iron-molded cylinder or a cast iron-molded cylinder which has at the inner area a gray cast iron structure and at the outer area the structure of white cast iron. Because of the size of the cylinder this cylinder of chilled cast iron is molded by bottom-casting with a tangentially flowing melt so that the structure of the cylinder is substantially rotation-symmetrical. The cylinder is then machined and drilled through.
Depending on the use, in operation of such a roller the latter is subjected either to low temperatures or cooled off or to high temperatures or thus heated. Operation temperatures of the roller are rather steady.
The rollers of the foregoing type are, however, not free of being unbalanced for many reasons. Concentricity of the limiting surfaces of the gray cast iron and white case iron is not achieved in practice.
As far as the rollers are not too long and do not rotate with high peripheral speed and act during the operation as approximately rigid rollers, these rollers are balanced-out according to the principle of dynamic counterbalance in two planes by means of compensating bores formed in lateral faces of the roller sleeve at respective places, and also in the heavy region of the roller.
However, such an imbalance-compensation is not sufficient when the roller exceeds a predetermined ratio of slenderness or a predetermined number of revolutions or both these criteria. With increased ratio of slenderness and increased number of revolutions a problem occurs as to where on the relatively long roller could be imbalance found. The roller under such conditions is no longer rigid during operation. If imbalance is found on one or two ends of the roller the above described balancing-out or compensation is sufficient. However, if imbalance is found at the substantial distance from the roller ends such a compensation is no longer sufficient. The roller than has a tendency to be bent or deflected in operation under the influence of centrifugal forces so that imbalance with the increased number of revolutions would be further increased. To counteract or inhibit that tendency the roller outer sleeve is precisely ground over and rotated with operation number of revolutions. During the rotation with operation number of revolutions a middle value of deflection or bending of the roller and also a value of deflection due to dynamic centrifugal forces are measured before the imbalance of the roller body is compensated for by means of bores formed at the ends of the roller body. This deflection of the roller body at the state of operation is compensated because the roller body is again polished or ground. The axis of rotation of the roller is so selected that no middle deflection occurs in the middle region of the roller body. This balancing-out centering process is completed with a commonly known procedure of balancing via dynamic counter-balance in two planes at the ends of the roller body. The centering step contributes to the deformations of the roller during the heating thereof and also to the bimetal effect.
The counterbalancing of the roller obtained by bores formed in the roller can be, however, easily lost. Even a minimal displacement of the center of the bore can cause a new imbalance which would lead to a new deflection of the roller having a great ratio of slenderness.